(1) Field of the Invention
The present invention relates to a novel aqueous dispersion type thermosetting coating composition. More particularly, the present invention relates to an aqueous dispersion type thermosetting coating composition which is excellent in storage stability and free of yellowing and which provides a coating excellent in gloss, smoothness, flash rusting prevention, corrosion resistance and water resistance without such defects and popping when it is baking temperatures of a relatively broad range.
(2) Description of the Prior Art
Recently, development of paints and coating methods causing no environmental pollution has been desired, and the use of powdery paints and aqueous paints has been expanding steadily. These paints have important characteristics and merits, but they still include various defects and disadvantages as well.
More specifically, the use of powdery paints involves various difficulties. For example, special coating equipment must be used for performing the coating operation using powdery paints, and hence, the adaptability of powdery paints to the coating operation is much lower than that of conventional solvent type paints. Moreover, coatings formed from these powdery paints are inferior in smoothness and appearance, and it is difficult to form thin coating films having a thickness smaller than 50.mu..
In the field of aqueous paints of the water-soluble type, since it is impossible to form paints having a high solid concentration, it is impossible to form thick coatings by one coating operation. Furthermore, because of the presence of a number of hydrophilic groups introduced to render the base resin water-soluble, the durability of the resulting coatings is very poor.
Paints of the aqueous dispersion type, called "latex paints" or "emulsion paints" have long been known (see, for example, U.S. Pat. No. 2,904,523, No. 2,994,676, No. 3,002,940, No. 3,025,252, No. 3,170,888, No. 3,244,542, No. 3,657,175, No. 3,687,885, No. 3,839,254 and No. 3,926,874). In these aqueous latex or emulsion paints, since surface active agents are used in large quantities, the resulting coatings are insufficient in water resistance, chemical resistance and the like.
As paints overcoming these defects and disadvantages, there have been developed slurry paints (aqueous dispersion type paints) which are formed by suspending a powdery resin in water and are used in the same manner as aqueous paints.
Paints of this type comprise an aqueous dispersion medium and a particulate resin having a size of about 0.5 to about 80.mu., which is dispersed in the aqueous medium at a solid concentration of 20 to 70% by weight.
As the coating method for these slurry paints, there can be mentioned, for example, a method in which the paint is spray-coated on the surface of an article to be coated, water is evaporated and the resin component is cured to form a coating (see U.S. Pat. No. 3,787,230) and a method in which an aqueous dispersion of particles of a nylon resin is spray-coated on an article maintained at a temperature higher than 100.degree. C. to form a coating (see U.S. Pat. No. 2,972,552). As known paints to be used for these coating methods, there can be mentioned, for example, (i) a paint comprising particles of a chlorinated polyester resin having a size smaller than 200 mesh, which are dispersed in water (see U.S. Pat. No. 3,089,783), (ii) a paint comprising particles of a chlorinated polyester polymer dispersed in water containing a surface active agent, an ester of a polyhydric alcohol with a lower aliphatic monocarboxylic acid, an alkaline water softener and a lower aliphatic alcohol (see U.S. Pat. No. 3,446,652), (iii) a slurry paint comprising particles of a resin selected from acrylic resins, epoxy resins, polyester resins, amino resins and polyurethane resins and a water-soluble thickener (see U.S. Pat. No. 3,907,737), (iv) a slurry paint comprising as the binder particles of a powder thermosetting acrylic resin (see U.S. Pat. No. 3,904,795), (v) a slurry paint comprising a reactive flow adjusting agent having a specific structure (see U.S. Pat. No. 4,122,055) and (vi) a slurry paint comprising an aqueous medium, resin particles having a size of 0.5 to 80.mu. and an aluminum pigment (see British Pat. No. 1,517,834).
As the process for preparing these slurry paints, there are known a process comprising dissolving a resin in a water-soluble solvent, for example, an alcohol such as methanol or ethanol, a ketone such as acetone or diacetone alcohol, an ester such as ethyl lactate or ethylene glycol monomethyl ether acetate or an ether such as ethylene glycol monoethyl ether, and adding the resulting solution to water being agitated at a high speed to precipitate resin particles in water (see U.S. Pat. No. 3,737,401), and a process comprising pulverizing a solid resin according to an ordinary pulverization method and dispersing the pulverized resin in water.
Slurry paints prepared according to these processes are characterized in that they can be applied at a solid concentration as high as about 40 to about 60% by weight, the coating operation can be accomplished by using a conventional coating apparatus for solvent dilution type paints and the only volatile component contained in the paint is the water that is used as the dispersion medium. Furthermore, since this aqueous dispersion medium contains no soluble component, conserving of resources can be attained and environmental pollution can be prevented. Because of these advantages and merits, it is expected that demands for paints of this type will increase remarkably from now on.
As the resin component that is used for these slurry paints, there are known acrylic resins, polyester resins, epoxy resins and alkyd resins. From the viewpoints of storage stability of paints and smoothness and finish appearance of the resulting coatings, acrylic resins and polyester resins are considered preferable among these resins. According to the type of the cross-linking reaction, these resins are classified as self-curing resins and resins cross-linkable with the aid of a cross-linking agent or curing agent. As the cross-linking or curing agent, there have heretofore been used melamine resins and blocked isocyanate compounds. Furthermore, epoxy resins are sometimes used in combination with such cross-linking or curing agents so as to improve the corrosion resistance.
When slurry paints comprising a melamine resin as the cross-linking agent are used, in case of thick coatings, popping is caused at the baking step and the adaptability to the coating workability is poor. In this case, therefore, the coating operation should be repeated several times to obtain a coating which is as thick as desired. The combined use of such melamine resin with a urethane (blocked isocyanate compound) has been proposed as means for preventing popping, but this proposal is defective in that yellowing is readily caused when baking is carried out at a high temperature (see Japanese Patent Application Laid-Open Specifications No. 112428/75 and No. 108433/77).
Since slurry paints comprise water as the dispersion medium, if they are directly applied to an iron plate such as a soft steel plate, as in the case of conventional aqueous paints, an undesirable phenomenon of flash rusting which is not observed in solvent type paints, is caused. In order to prevent this undesirable phenomenon, a flash rusting-preventing agent or the like is frequently incorporated into slurry paints as in the case of aqueous paints. However, the storage stability or the like is adversely affected by the use of such flash rusting-preventing agent and the use of such flash rusting-preventing agent is not preferred.